DIFFERENT PATHWAYS OF SECONDARY PHASE FORMATION INDUCED BY COLLOIDAL AND DISSOLVED SILICA DURING THE DISSOLUTION OF UO_2 NUCLEAR FUEL IN LEACHING TESTS

摘要

We investigated the different dissolution behaviour of UO_2 nuclear fuel material in waters containing silica in two different physical and chemical forms (dissolved ions and as SiO_2 colloids, respectively) at elevated temperatures (95°C in autoclaves). It was investigated if SiO_2 colloids can act as carrier material for U ions during a interface geochemical dissolution process, a process that might possibly enhance the mobilization of uranium. Herefore, leaching / dissolution tests were conducted in batch reactors, using both dissolved Si (sodium metasilicate solution), as well as synthetic SiO_2 colloids (100 nm diameter). Solid materials were examined with scanning electron microscopy (SEM-EDX) after the tests and ICP-OES was used for analysis of concentrations of U and Si in solutions. Thermodynamic calculations were applied for modelling the surface charges of the solid materials. Results show that a treatment with colloidal SiO_2 has different effects on the surfaces than a leaching in dissolved silicate solutions. In the presence of colloids, well-crystallized secondary phases containing U and Si (most obviously uranyl silicates) were found on the surfaces, which were attacked by the treatment. This was not the case when dissolved Si was used. SiO_2 colloids were partly found to remain on the surfaces after 1000 h at 95°C. Dissolved U concentrations decreased with increasing Si content in the systems, especially so when colloidal Si was used. Ultrafiltration showed that the greatest part of the dissolved U was associated with Si colloids. A surface charge model suggests that the different effects are due to the development of electrostatic interactions between the UO_2 and SiO_2 surfaces.